I've got a similar setup, but use an Ubuntu laptop running docker with a Joplin server container. I've got the APP_BASE_URL set as http://hostname:22300, and it works well. You do have to make sure your client can resolve hostname to the IP address of the Synology. I think there are various ways; hosts file, your router, maybe a pihole (very much recommended as another container on your synology) or perhaps your own DNS server if you're looking for something else to learn/set up.
For external access, I use OpenVPN into the same Ubuntu laptop, and client can then also sync using http://hostname:22300 as the sync target.
Works beautifully, so thanks to Team Joplin!
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